1. Technical Field
This technique relates to a disk array apparatus that includes multiple hard disk drives constituting a RAID (redundant arrays of inexpensive disks) group for creating parity for each stripe and controls access requests to hard disk drives made by the host, a disk array apparatus control method, a disk array apparatus control program, and a disk array controller. In particular, the technique relates to a disk array apparatus, a disk array apparatus control method, a disk array apparatus control program, and a disk array controller that each improve the capability to perform read processes on hard disk drives using a simple process without running out of disk space.
2. Description of the Related Art
Large-scale computer systems have managed data using a dedicated data input/output unit provided independently of the host computer. Among such data input/output units, a disk array apparatus includes multiple hard disk drives (storage media), which constitute a RAID group. Thus, the reliability of data handled by the apparatus as well as the apparatus's capability to access the hard disk drives is improved.
RAID is classified into RAID levels corresponding to the levels of rapidity or fault tolerance. One of RAID levels being used commonly at present is RAID 5 . RAID 5 is intended to switch between hard disk drives that are assigned the storage of an error correcting code called “parity” and hard disk drives that are assigned the storage of data, for each stripe. Thus, a disk array apparatus in which RAID 5 is implemented improves fault tolerance, increases the capacity, and speeds up read processes.
As the amounts of data handled by computer systems are increased in recent years, the amounts of data held by disk array apparatuses have also been increased. Such a disk array apparatus is often accessed intensively. Although RAID 5 allows a disk array apparatus to speed up read processes, the apparatus's capability to perform read processes is naturally reduced if the disk array apparatus is intensively accessed. As the information technology progresses day by day, computer systems including a disk array apparatus are always required to enhance their capabilities. Under the circumstances, disk array apparatuses are required to sufficiently demonstrate their capabilities even if they are intensively accessed.
In view of the foregoing, a technology regarding an improvement in a capability of a disk array apparatus is disclosed in Laid-open Japanese Patent Application Publication No. 2003-150324. Specifically, a technology for improving the capability to perform read processes by providing a swap area in each RAID group and saving a frequently accessed data block in the swap area is disclosed in Japanese Laid-open Patent Publication No. 2003-150324.
However, the technology disclosed in Japanese Laid-open Patent Publication No. 2003-150324 has a problem that the disk space is pressed and a problem that a complicated process must be performed. Specifically, the technology disclosed in Japanese Laid-open Patent Publication No. 2003-150324 has a problem that since a swap area is provided for each RAID group, the disk space for storing user data is reduced and a problem that since the access frequency is monitored periodically by counting the number of accesses for each data block, the number of processes is increased so that the processing capability as a whole may be reduced.
In view of the foregoing, a significant challenge to a disk array apparatus in which RAID 5 being used commonly at present is implemented is to improve the capability to perform read processes using a simple process without running out of disk space.